1) Field of the Invention
The present invention relates to an optical recording medium which carries out recording by irradiating condensed laser beams on a substrate fitted with a recording layer containing a heat-sensitive recording material and a light absorbent which absorbs light having a central wave length in the near infrared region, performs thermal conversion of light energy and emits in the form of heat energy.
2) Description of the Related Art
The heat-sensitive recording system is a direct recording system which does not require development and fixing steps, and is excellent in operation and maintenance. Hence, the system is widely used for facsimiles and printers.
In this system, however, hot recording is conducted by bringing a thermal head or an exothermic IC pen into direct contact with a heat-sensitive recording paper. Consequently, colored substances in a fused state adhere to the thermal head or the exothermic IC pen, cause troubles such as dregs adhesion and sticking, and lead to problems of record obstruction and impairment of record quality.
In particular, when a line is continuously drawn in a recording direction as in the case of a plotter printer, it has been impossible to carry out continuous printing without trouble of dregs adhesion.
Further, in the recording system using the thermal head, it has been considered difficult to heighten an image resolution to 8 dott/mm or more.
Consequently, non-contact recording systems using light have been proposed as techniques for eliminating the troubles of dregs adhesion and sticking and further improving image resolution.
Japanese Patent Laid-Open No. 209594/1983 discloses an optical recording medium prepared by laminating on a substrate at least one set of layer consisting of a near infrared absorbent layer having an absorption wave length in a near infrared region of 0.8 to 2 .mu.m and a heat-sensitive color-developing material layer. Japanese Patent Laid-Open No. 94494/1983 describes a recording medium obtained by coating a substrate with one or more of heat-sensitive color-developing materials and one or more of near infrared absorbents composed of compounds having a maximum absorption wave length in the near infrared region of 0.7 to 3 .mu.m.
In these recording media, application of the near infrared absorbent and the heat-sensitive color-developing material to a substrate or a base material is carried out by mixing the near infrared absorbent and the heat-sensitive color-developing material and applying the resulting mixture, or by previously applying a heat-sensitive color-developing material layer on the substrate or the base material and then applying the near infrared absorbent on the layer of the heat-sensitive color-developing material to form a laminated layer.
In these publications, cyanine dyes, thiol nickel complexes and squalium dyes have been disclosed as the near infrared absorbents having absorption wave length in the near infrared region of 0.8 to 2 .mu.m. Other near infrared absorbents which have been known are nitroso compounds and metal complexes thereof, polymethine dyestuffs (cyanine dyestuffs), cobalt or palladium complexes of thiol, phthalocyanine dyestuffs, triallyl methane dyestuffs, immonium or diimmonium dyestuffs and naphthoquinone dyestuffs. These near infrared absorbents are described in the paper entitled "Near Infrared Absorption Dyestuffs" [Chemical Industry (Japan), 43, May 1986].
Many of the above known near infrared absorbents have been developed for use in a high density memory such as optical discs. These absorbents are laminated on a disc substrate in the form of a uniform membrane and are hence required to dissolve in organic solvents. Consequently, these near infrarred absorbents could not be used by dissolving in an aqueous system.
On the other hand, those of absorbents being dispersed in the aqueous system has been proved to be excellent as a method for constructing the heat-sensitive recording medium in view of safety and mass production.
Accordingly, development of an excellent near infrared absorbent having solubility in water is inevitable for the preparation of an optical recording medium constituting of the heat-sensitive recording material and the near infrared absorbent.
Any of the above conventional near infrared absorbents have absorption in a visible region to a certain extent. As a result, these absorbents are colored and cause problems of exhibiting undesirable color tone when these absorbents are used for the optical recording medium.